The present invention relates to a method for filling foam-structure or fiber-structure electrode plaques with pastes of active substance for galvanic cells, particularly for electrical accumulators, as well as to a composition of the pastes of active substance.
Electrode plaques having a foam or fiber structure are being increasingly widely used because they present advantages compared to the cheap cast and expanded metal grids as regards electrical load capacity and useful life. The fibers of these plaques may consist entirely of metal, such as nickel-plated steel wool for example, or they may be formed by metallization of plastic non-woven fabrics, according to U.S. Pat. No. 3,560,262 for example.
Foam-structure plaques, which are also frequently known as reticulated, plaques, can be produced by the metallization of open-cell foam materials such as carbonized foam materials according to Great Britain Patent Specification 1,211,428 for example, or of foam materials according to German Published, Unexamined Patent Application (DE OS) 2,427,422 or by foaming a plastic material containing a metal powder and subsequent pyrolysis according to European Patent Specification (EP-PS) 87,160. For reasons of production, the pores of these reticulated plaques form polyhedra of approximately spherical shape and vary far less in shape and size than the pores of plaques which consist of non-woven textile materials. Due to the gas bubble sizes dictated by the process technology, the pore diameter of the reticulated plaques is much greater than that of sintered, plaques produced by powder metallurgy, and a considerable reduction in the thickness of the reticulated plaques is generally undertaken by rolling or pressing. The plastic fraction is almost always removed from the reticulated plaques by pyrolysis, so that all-metal plaques with no plastic fraction are obtained in the final result.
The following classification of electrode plaques categories by pore sizes and pore shapes can be made
(a) Very large pores in the millimeter range in the form of bi-dimensional holes regularly arranged Cast grids, expanded metal grids, nets.
(b) Large pores, polyhedra approximating to spherical shape, pores cross-linked three-dimensionally, narrow pore-size distribution: Reticulated (foam) plaques.
(c) Medium-sized pores, anisodimensional (widely different dimensions of a pore in different Physical directions), pore delimitation by partially elongate bars, three-dimensional cross-linkage of the pores: fiber plaques and compressed foam materials.
(d) Small pores, three-dimensionally cross-linked, pore delimitation by sintered necks, approximately 60% of the total pore volume in the form of pores in the range 4 to 15 micrometers (Falk and Salkind, Alkaline Storage Batteries, Publisher J. Wiley 1969, Page 122): powder metallurgical sintered plaques.
Various techniques which are oriented by the above enumerated pore characteristics, have been developed for the introduction of the active mass by filling or impregnation.
The filling of the plaques mentioned under category (a), which are provided with two-dimensional, optically transparent hole patterns, is generally done by pressing in a paste of active mass mechanically, as for the electrodes of lead accumulators, where this technology has been applied to cast grids and expanded metal grids. Rollers or doctor blades are used pressure-imparting elements. Since the active mass paste be sufficiently firm not to flow out of the holes after pasting, but sufficiently plastic to be still spreadable, high viscosity thixotropic pastes are used (e.g. German Patents Specifications (DE-PS) No. 2,517,368 or (DE-PS) No. 2,602,904) which become sufficiently fluid under the pressure of the roller or of the doctor blade.
For narrow-pored plaques with a three-dimensional pore network, this method becomes more difficult the narrower the pores are. Only chemical or electrochemical precipitation methods of embedding the active masses have therefore been successful for sintered plates, because the solutions can penetrate even into very small pores. However, the methods which operate with solutions suffer from disadvantages of tediousness, accompanying frame corrosion and also contamination of the precipitation products by anions of the dissolved compound, nitrate, sulphate or chloride ions being typical for example. The tediousness of the filling is due, in principle, to the much lower ion concentration in the solutions as compared to the solids, so that much time is required to build up the desired high solid concentration of the active mass in the pore volume of the electrode.
Both the precipitation impregnations known for category (d) and also mechanical filling methods with suspensions and pastes have been described for the foam-structure and fiber-structure plaques mentioned under categories (b) and (c). Naturally, the less expensive mechanical methods encounter increasing difficulty of realization with a reduction in the pore size, because suspensions and pastes do not penetrate into a network of fine pores as easily as solutions.
A filling with a paste of active mass with agitation is described for reticulated plaques with large pores between 230 and 2,540 micrometers in German Patent Specification (DE PS) No. 1,596,023 (which correspond to U.S. Pat. No. 3,287,164). The liquid is 30% caustic soda solution. No information is given as to the viscosity of the paste, the agitation conditions (frequency, intensity, arrangement). However, the filling of such large pores presents no problems.
The filling of a foam plaque with a somewhat smaller pore diameter of 100 to 500 micrometers without metallization is described with similar terseness in German Published, Examined Patent Application (DE-AS) No. 1,108,759. According to this document an active mass placed in suspension is pressed in, optionally with agitation. However, according to the description as a slurry the active mass suspension cannot be a paste.
An apparatus for the impregnation of reticulated plaque (foam metal) with paste, likewise with no disclosure concerning the details of the pore size, is described in U.S. Pat. No. 4,217,939. The plaque is guided horizontally on a perforated plate over a paste tank, in which the paste is moved by stirrers and pressed upwards into and round the plaque, the paste being swept into the plaque from above by reciprocating doctor blades. However, it is not immediately possible then to displace the air out of the pores of the plaque by the paste. On the contrary, a preimpregnation of the plaque with water is necessary for this purpose. The dilution of the paste with this water creates difficulties in adjusting the concentration of the paste.
The willing of fiber-plaque electrodes with paste is described in a plurality of applications.
U.S. Pat. No. 3,262,815, corresponding to Great Britain Patent Specifications 1,109,524, describes three different methods for introducing a suspension of an active mass into a fiber plaque: (1) working in mechanically, (2) depth filtration and (3) introducing the plaque into a mechanically moved (agitated) bath, where the plaque or the bath may be set in motion. None of these three methods operates satisfactorily by itself, because the disclosure is directed at a process which should combine all three methods. Thus, after treatment in accordance with method step (3) noted above, the plaque should still undergo an after treatment according to method steps (2) and (1). No information is given as to the mechanical characteristics of the paste; however, the treatment according to method step (3) alone clearly does not provide a satisfactory filling.
Then again, in a later application, German Published, Unexamined Patent Application (DE-OS) No. 2,436,704, the same applicant refers to the basic process of working in a thixotrophic paste by pressure roller application, as already described above for lead grids, except that the method is applied to nickel fiber plaques of 92% porosity and 1.7 mm thickness (Example 1). After filling, these plaques are compressed to 0.9 mm thickness. This high compression, which for an empty, i.e., unfilled plaque wood reduce the porosity from 92% to 85%, indicates that a paste filling into the compressed plaques of final thickness clearly does not succeed, a double paste application being required even for the still uncompressed frame of 92% porosity
German Published, Unexamined Patent Applications (DE-0S) No. 2,427,421 (which corresponds to U.S. Pat. No. 3,877,987) and (DE-OS) 2,427,422 (which corresponds to U.S. Pat. No. 3,926,671) refer in identical texts to the possibility of filling fiber plaques with suspensions of active substance which are pourable and therefore highly fluid. The suspension is poured onto the horizontally positioned plaque and a vibrator electrode arranged parallel to the plaque, which is in communication with an ultrasonic generator, and in combination with vacuum ensures a substantial filling, which however still requires completion by a following precipitation impregnation. According to the descriptions in these applications, the suspensions of active masses are not pastes; nor is the term paste used.
The specifications discussed above refer to plaques which must be classified under a specific category (a) to (c) of the above-mentioned plaques classification according to the pore size. However, a vibration filling with pastes under the influence of ultrasonic vibrations, and optionally of vacuum, is disclosed by German Published, Examined Patent Application (DE-AS) No. 1,287,663, which is claimed to be useful both for fiber plates of category (c) and also for sintered plaques of category (d) produced by powder metallurgy, By this filling method the paste is applied in a thin uniform layer onto the ultrasonic vibrator, and a pressure element presses the paste layer together with the plaque strongly against the vibrator, a perforated foil being placed between electrode plaque and pressure element. No detailed information is given as to the flow characteristics of the paste, except for the example, according to which the paste should have a consistency somewhat like spreadable butter.
The horizontal positioning of the plaque, which is adopted for all filling processes under the influence of vibrations, is considered unavoidable because in the vibration field the dispersed solid has a tendency to settle downwards, whereas the carrier liquid moves upwards (See German Published, Examined Patent Application (DE AS) No. 1,287,663). However, considerable disadvantages are associated with the horizontal positioning. It is known for instance for the filled plaque to stick very firmly to the pressure plate or to the surface of the vibrator, which has the result that special removal technologies were proposed (See German Patent Specifications (DE-PS) No. 1,210,417 for example) or that the corresponding parts threatened by sticking are provided with an antiadhesion coating or with an antiadhesion foil. Furthermore, the production process according to the two last mentioned patents, whereby the plaque to be filled is sandwiched horizontally between two layers of the active mass and is then filled under pressure and the influence of vibrations, is naturally also extraordinarily complicated.
To sum up, it can be stated that in the use of pourable suspensions, such as they are described in German Published, Examined Patent Application (DE-AS) No. 1,108,759, German Published, Unexamined Patent Application (DE-OS) No. 2,427,421, U.S. Pat. No. 3,262,815, U.S. Pat. No. 3,877,987 or U.S. Pat. No. 3,926,671 for example, a single filling operation does not effect an adequate filling of the poreds, so that a plurality of impregnation passes or following impregnations are recommended.
In the use of pastes of high solids content, i.e. pastes being understood to mean, according to Rompp, Chenielexikon 8th Edition 1985, Volume 4, Page 3006, solid liquid dispersions of doughy consistence, and therefore not pourable suspensions, the difficulties known from US Pat. No. 4,217,939 and German Published, Examined Patent Application (DE-AS) No. 1,287,663 as described above result.
An object of the present invention is to provide a method for the filling of foam-structure or fiber-structure electrode plaques with a paste of active mass which is simple, fast and can be performed without a major outlay of apparatus or effort.
A further object of the present invention is to provide a paste of active mass suited for the method of filling the electrode plaques of the present invention.
Thus, in accordance with preferred embodiments of the present invention the electrode plaques to be filled are immersed in a tank filled with a paste of active mass, in which the paste is set in vibration. The paste of in the tank has an active mass content of about 25 to 60 vol%, with a maximum granulomery of the active mass of about 0.04 mm and a yield value of about 10 to 120 Pa at about 20.degree. C. and a plastic viscosity of about 0.1 to 1 Pas at about 20.degree. C. The viscosity particulars here refere to a paste which is not in vibration. This is because a reliable viscosimetry is not known for a vibrating paste.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.